This invention relates to radio transmitters.
In order to limit the potential for interference between radio systems that occupy nearby or overlapping frequency bands, regulatory authorities stipulate limits on the radiation that can be transmitted from certain transmitters. Those limits may include upper and lower frequency bounds, an upper instantaneous power limit or an upper time-averaged power limit.
Among a batch of mass-produced radio transmitters there may be variation in performance due to process variations during manufacture. In particular, the size of the power amplifying transistors can vary with a range of around ±2 dB. The performance of devices also varies with temperature. These factors lead to differences in transmit power between apparently similar devices. In order to ensure that all of a manufacturer's devices will meet regulated power limits, the manufacturer will normally design his transmitters to operate somewhat below the power limits so that even those devices that are at the extremes of performance will still comply with the regulations.
The UWB radio system has especially strict power limits. The regulations for the UWB radio system require that the transmitted power from a UWB device must not exceed −41 dBm/MHz averaged over any 10 ms timeframe. The UWB protocol uses 1.5 GHz bands, so that regulated power limit equates to −10 dBm over the 1.5 GHz band. That is a very low figure compared to typical radio systems. The UWB power limit is so low that if a manufacturer were to adopt the normal approach of designing products to a significantly lower power limit than the regulated limit, the resulting drop in power output would result in a serious reduction in the devices' usable range. There is therefore a need for an alternative way of ensuring that the power limit is met.
One way to increase the effective power of UWB devices is to design them to tighter tolerances. This reduces the leeway that is needed below the power limit in order to ensure that the devices meet the power requirement. However, this approach increases the cost of the devices.
Another way to increase the effective power is to calibrate each device at manufacture. Again, this increases the devices' cost.
Alternatively, it is known for devices to perform self-testing with the aim of regulating their own power. The difficulty with this approach is that in order for the manufacturer to be certain that the self-calibration process will result in the device meeting the legal power limits the self-calibration must be performed accurately. But for the same reasons that the transmitters' outputs can vary between devices, the accuracy of the components that perform calibration can also vary between devices. Also, the performance of both the transmitter and the calibration components can vary with temperature.
There is a need for an alternative way to allow a transmitter to run close to a regulated power limit, or in general to operate at a well-defined output power.